The present invention relates to road vehicle sensing apparatus.
In the prior art a known road vehicle sensing apparatus comprises at least one sensor for location in at least one lane of a highway to detect vehicles travelling in said lane. A signal generation circuit is connected to the sensor and is arranged to produce a sensor signal having a magnitude which varies with time through a plurality of values as a vehicle passes the sensor in said lane. When there is no vehicle near the sensor, the signal magnitude is at a base value. Apparatus of this type will be referred to herein as road vehicle sensing apparatus of the type defined.
The sensors used in road vehicle sensing apparatus of the type defined are typically inductive loops located under the road surface, which are energized to provide an inductive response to metal components of a vehicle above or near the loop. The response is usually greatest, providing a maximum sensor signal magnitude, when the maximum amount of metal is directly over the loop. Other types of sensor may also be employed which effectively sense the proximity of a vehicle and can provide a graduated sensor signal increasing to a maximum as the vehicle approaches and then declining again as the vehicle goes past the sensor. For example magnetometers may be used for this purpose.
In a multi lane highway, with two or more traffic lanes for a single direction of travel, it is normal to provide separate sensors for each lane so that two vehicles travelling in lanes side by side can be separately counted. The signal generation circuit is arranged to provide a separate said signal for each sensor. The sensors in adjacent lanes are usually aligned across the width of the highway. Apparatus of this type with adjacent sensors in the lanes of a multi lane highway will be referred to herein as road vehicle sensing apparatus of the type defined for a multi lane highway.
It is also normal practice for the sensor installation on a single lane of highway to include two sensors installed a distance apart along the lane of the highway. Again the signal generation circuit produces a separate said signal for each sensor. This is arrangement allows the direction of travel of a vehicle in the lane to be determined and also the timing of the signals from the two sensors can be used to provide a measure of vehicle speed. The first sensor in the normal direction of travel in the lane can be called the entry sensor and the second sensor can be called the leaving sensor. Apparatus of this type will be referred to herein as vehicle sensing apparatus of the type defined with two successive sensors in a single lane.
In the prior art, vehicle sensing apparatus of the type defined has been used primarily for the purpose of counting the vehicles to provide an indication of traffic density. Although the signal generation circuit of the apparatus of the type defined provides a sensor signal of varying or graduated magnitude, a typical prior art installation has a detection threshold set at a magnitude level above the base value to provide an indication of whether or not a vehicle is being detected by the sensor. Thus, in prior art installations, the only information available from the sensing apparatus is a binary signal indicating whether or not the sensor is currently detecting the vehicle, that is whether the sensor is xe2x80x9cdetectxe2x80x9d.
Prior art sensing apparatus using one or more inductive loops under the road surface have signal generation circuitry arranged to energize the loops at a frequency typically in the range 60 to 90 kHz. In some examples, a phase locked loop circuit is arranged to keep the energizing frequency constant as the resonance of the loop and associated capacitance provided by the circuit is perturbed by the presence of the metal components of a road vehicle passing over the loop. The sensor signal produced by such signal generation circuit is typically the correction signal generated by the phase locked loop circuit required to maintain the oscillator frequency at the desired value. In a typical circuit, the correction signal may be a digital number contained in a correction counter. As a vehicle passes the loop sensor, the digital number from the counter may progressively rise from zero count up to a maximum count (which in some examples may be between 200 and 1,000) and then falls again to zero as the vehicle moves away from the sensor loop. As mentioned above, prior art installations are arranged to set a threshold value for the sensor output signal, above which the sensor is deemed to be xe2x80x9cin detectxe2x80x9d.
The present invention in its various aspects is based on the realization that there is far more information available in the output signals of vehicle sensing apparatus of the type defined which can be employed so as to improve the reliability of the prior art installations.
Prior art installations are reasonably reliable and accurate in counting vehicles, so long as the traffic is free flowing along the highway with a reasonable spacing between vehicles, and so long as the vehicles do not cross from one lane to another in the vicinity of the sensor installation. In practice, however, a typical installation has a vehicle count accuracy of only about plus or minus one percent even in free flowing traffic conditions. In congested traffic conditions, count accuracy falls dramatically and is seldom specified.
There is an increasing need for more accurate automatic traffic monitoring. This need has been stimulated by proposals for highways to be maintained, or even constructed, with private finance, and compensation to be paid to the constructors/maintainers by Central Government or a Regional Authority in accordance with the number of vehicles using the highway. Even a 1% error in count accuracy would be too high. Importantly, also, the vehicle sensing apparatus should be capable of determining the class of the vehicles using the highway, usually on the basis of vehicle length. Also, the sensor should be able to provide accurate information even in congested conditions.
Various aspects and preferred embodiments of the present invention are defined in the appended claims.